A basic arrangement of color cathode ray tubes comprises an electron gun for emitting an electron beam and a phosphor screen for emitting light to develop an image when scanned by the electron beam. The electron beam may however be undesirably deflected by the effect of geomagnetism, hence causing color deviation in the image. For preventing such deflection, internal magnetic shields (also termed inner shields or inner magnetic shields) are installed. Additionally, external magnetic shields (also termed outer shields or outer magnetic shields) are provided outside the color cathode ray tube, in some cases. For simplicity, those inner magnetic shields and outer magnetic shields are referred to as magnetic shields hereinafter.
Recently, as commercial TV sets have been enlarged or widened in the screen size, the flight path length and scanning length of the electron beam increase significantly and thus TV sets have become more susceptible to the effect of geomagnetism. In other words, a deviation of the landing point on the phosphor screen of the electron beam from the designated point, which is caused by the effect of geomagnetism (thus termed a geomagnetic drift), may be increased more than ever before. Since higher definition in the still image is requested in a cathode ray tube used for a personal computer display, it is most crucial to reduce such color deviation caused due to the geomagnetic drift.
In this circumstance, steel sheets used for the magnetic shields are often evaluated on the basis of known parameters including the magnetic permeability in a low magnetic filed equivalent substantially to the geomagnetism, the coercive force, and the remanent flux density.
One of technologies for improving the characteristics of steel sheet for magnetic shields is disclosed in Japanese Patent Disclosure (KOKAI) No. 3-61330 where the ferrite grain size number in a specific composition steel is defined to not larger than 3.0 to improve the magnetic properties. It is also described in the same disclosure that the required magnetic permeability of not less than 750 G/Oe and the required coercive force of not more than 1.25 Oe are mentioned as examples of preferable magnetic properties for a cold-rolled steel sheet for magnetic shields.
Alternatively, disclosed in Japanese Patent Disclosure (KOKAI) No. 5-41177 is a technique for producing an inner magnetic shield using of a magnetic material of which the remanent flux density is not less than 8 kG.
In Japanese Patent Disclosure (KOKAI) No. 10-168551, an improved magnetic shielding material which is used a specific composition steel of which the grain size of the product is kept small and having the coercive force of not less than 3 Oe and remanent flux density of not less than 9 kG, and a manufacturing method thereof are disclosed.
As those conventional technologies are unsatisfactory in the magnetic shielding effect, they may hardly overcome degradation in the image quality caused by color deviation pertinent to advanced commercial TV sets with the enlarged and/or widened screens. It is highly desired to provide improved steel sheets for magnetic shielding which have a higher level of the magnetic shielding effect.
In an article, Transaction (in Japanese) of the Institute of Electronics, Information, and Communication Engineers, vol. J79-C-II No. 6, p. 311–319, June 1996, the relationship between anhysteretic magnetic permeability and magnetic shielding effect is described, and it is pointed out that the higher the anhysteretic magnetic permeability is, the higher the magnetic shielding effect becomes.
The article, however, only describes the relationship between anhysteretic magnetic permeability and magnetic shielding effect, and it fails to disclose which type of steel sheet has a higher level of the anhysteretic magnetic permeability.